feat(ui): phone layout up to 768 px

Phones in landscape and small tablets (iPad mini upright) got the
desktop layout from 640 px. The sm breakpoint is now 768 px; the media
queries in index.css and the width checks in code (isPhoneWidth) follow
it. The desktop header grid applies from sm.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

frontend/src/features/workspace/WorkspaceView.tsx

@@ -60,6 +60,7 @@ import { CountBadge } from '../../components/CountBadge';
 import { EmojiIconButton } from '../../components/EmojiIconButton';
 import { LazyMount } from '../../components/LazyMount';
 import { extractHashtags } from '../../lib/hashtags';
+import { isPhoneWidth } from '../../lib/breakpoints';
 
 interface WorkspaceViewProps {
   currentUser?: User | null;
@@ -624,7 +625,7 @@ export const WorkspaceView: React.FC<WorkspaceViewProps> = ({
   const [newNoteIcon, setNewNoteIcon] = useState('🗒️');
   const openNewNote = useCallback(
     (folderPath = '') => {
-      if (window.innerWidth >= 640) {
+      if (!isPhoneWidth()) {
         void handleCreateDocument(folderPath);
         return;
       }